1. Field of the Invention
The present invention relates to an interferometer, a demodulator, and a receiver-transmitter.
2. Description of the Related Art
In the field of optical transmission, as an optical modulation scheme suitable for increasing transmission capacity compared to an intensity modulation scheme in the related art, the utilization of a phase modulation system, such as differential binary phase shift keying (DPSK) or differential quadrature phase shift keying (DQPSK), has been studied.
A light signal modulated by the DPSK scheme, the DQPSK scheme, or the like is demodulated by a demodulator including at least one delay interferometer. As the implementation of the delay interferometer, there are primarily an implementation in which an optical waveguide is used and an implementation in which a space optical system using a bulk optical element is used. In the former, temperature control is needed, causing high power consumption. In the latter, low power consumption can be achieved. For this reason, the latter form is attracting attention as a dominant mounting form.
FIG. 1 is a plan view of a Michelson interferometer which is frequently used as a delay interferometer of space optical system. As shown in FIG. 1, in the Michelson interferometer, a half beam splitter 103 and light reflective elements 106 and 107 are disposed on a substrate 101. Light 102 to be measured branches into a branched light beam 104 and a branched light beam 105 by the half beam splitter 103. The branched light beams 104 and 105 are respectively reflected by the light reflective elements 106 and 107, become a reflected light beam 108 and a reflected light beam 109, and are guided to the half beam splitter 103 again. The reflected light beam 108 and the reflected light beam 109 are combined by the half beam splitter 103 and become interference light beams 110 and 111. The light reflective elements 106 and 107 are disposed such that the difference in the optical path length between the branched light beam 104 and the branched light beam 105 corresponds to one symbol of the modulated light beam signal. As the light reflective elements 106 and 107, a right angle prism, a corner cube prism, or the like is used.
In the Michelson interferometer, if the optical axes of a branched light beam before and after being reflected by the light reflective element overlap each other, then it becomes impossible to detect one of the two generated interference light beams, and furthermore, laser oscillation becomes unstable because a portion of light to be measured returns to a light source.
For this reason, as shown in FIG. 1, in general, the optical axes of a branched light beam before and after being reflected by the light reflective element differ in the direction parallel to the substrate (for example, see FIG. 3 of JP 2006-53049 A).
In JP 2005-525539 A, FIG. 10 shows an interferometer in which light emitted from a polarization branch element is moved in parallel in a direction at 45 degrees with respect to a plane, on which an optical component is disposed, by a retroreflective element, and is guided to the polarization branch element again.
In the interferometer of the related art, the size of an optical component, such as a half beam splitter, has to be increased depending on the interval between the optical axes of a branched light beam before and after being reflected by the light reflective element, making it difficult to reduce the size of the entire interferometer.